Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce objects through layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is formed across a powder bed in a build chamber by depositing a heap of powder adjacent to the powder bed and spreading the heap of powder with a wiper across (from one side to another side of) the powder bed to form the layer. A laser beam is then scanned across areas of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. An example of such a device is disclosed in U.S. Pat. No. 6,042,774. Layers can be formed through rotational motion of a wiper relative the build platform, as described in U.S. Pat. No. 8,172,562.
The strategy used for scanning a laser beam can affect the thermal loads generated during the build and accuracy of a resultant solidified line of material. Excessive, unrestrained thermal loads created during the build cause warping and/or curling of the part being built.
Typically, the laser beam is scanned across the powder along a scan path. An arrangement of the scan paths will be defined by a scan strategy.
U.S. Pat. No. 5,155,324 describes a scan strategy comprising scanning an outline (border) of a part cross-section followed by scanning an interior (core) of the part cross-section. Scanning a border of the part may improve the resolution, definition and smoothing of surfaces of the part.
U.S. Pat. No. 5,155,324 and US2008/0241392 A1 describe scanning an area in a plurality of parallel scan paths (hatches). (Referred to herein as a “meander scan”). The direction of the scan paths are rotated between layers to homogenize tensions generated during the build. US2008/0241392 A1 extends this concept to scanning in a series of parallel stripes, wherein each stripe consists of a plurality of parallel scan path (hatches) running perpendicular to a longitudinal direction of the stripe. (Referred to herein as a “stripe scan”). US2008/0241392 A1 also discloses the stripes covering partial regions (commonly squares) of the area to be solidified, wherein the hatches of adjacent partial regions are at 90 degrees to each other. (Referred to herein as a “checkerboard scan”)
US2005/0142024 discloses a scan strategy for reducing thermal loads comprising successively irradiating individual areas of a layer, which are at a distance from one another that is greater than or at least equal to a mean diameter of the individual areas. Each individual area is irradiated in a series of parallel scan paths (hatches).
It is known to use a continuous mode of laser operation, in which the laser is maintained on whilst the mirrors move to direct the laser spot along the scan path, or a pulsed mode of laser operation, in which the laser is pulsed on and off as the mirrors direct the laser spot to different locations along the scan path.
A melt pool generated by the laser is dependent upon the properties of the material and the state (powder or solidified) and temperature of material surrounding the volume being melted. The mode of laser operation can affect the state and temperature of the neighbouring material. For example, scanning of the laser spot along a scan path in continuous mode forms a large melt pool that is dragged along just behind the laser spot, resulting in larger, less detailed solidification lines. For some materials, such as tool steels and aircraft grade super alloys, it can be difficult to drag the melt pool across the layer in a continuous mode of operation of the laser. These problems can be mitigated by using the laser beam in the pulsed mode of operation. In particular, setting the time between pulses/distance between exposure points to be long enough to allow a previously formed melt pool to cool before forming an adjacent melt pool can result in more accurate solidification lines, which may be particularly beneficial for border scans.
The laser beam is typically steered on to the powder bed using a pair of mirrors that can be tilted to a required angle by galvanometers. An example of such a scanner is described in WO2010/026397. The limited dynamic response of the galvanometers can result in significant divergence of the laser beam from the desired patterns (as described above). In particular, it has been found that, at the transition between hatch lines, the mirrors direct the laser beam in a curved path, visible in parts that are manufactured, rather than providing a sharp angular change in direction. For scanning of a laser in a pulsed mode, ideally the mirrors direct the laser beam at a fixed spot on the working surface when the laser is on, hopping to the next exposure point between pulses. However, the dynamic response of the galvanometers is too slow relative to desired point exposure times and time between exposures such that the mirrors will still be moving during an exposure. This results in the creation of elongate melt pools having melt characteristics similar to continuous scanning. Such problems can be overcome by increasing the time between exposures in order to give the mirrors time to move to and settle in the new position but this can significantly lengthen the build time.
“Electro-optic and acousto-optic laser beam scanners”, G. R. B. E. Römer, P. Bechtold, Physics Procedia 56 (2014), 29-39, discloses combining a mirror based scanner with an electro or acoustic optical deflector, which relies on a change in refractive index n of the material to control deflection of a laser beam. A problem with the electro and acoustic optical deflectors is that the deflectors absorb a significant proportion of the light travelling therethrough. Accordingly, such deflectors are unsuitable for use with high power laser beams because of the resultant heating of the deflector that would occur. Such heating would make it extremely difficult, if not impossible, to control the temperature of the crystals of the deflector to be just above the Curie temperature, as required.